Along with the recent tendency toward higher-grade color television sets, a 36 wt. % Ni-Fe alloy known as the INVAR alloy, which is a low-expansion alloy containing 36% nickel, 0.35% manganese and the balance iron with carbon, alloy is attracting the general attention as an alloy for a shadow mask capable of coping with problems such as a color-phase shift. The INVAR alloy has a far smaller thermal expansion coefficient as compared with a low-carbon steel conventionally applied as a material for a shadow mask.
By manufacturing a shadow mask from the INVAR alloy, therefore, even when the shadow mask is heated by an electron beam, there hardly cause such problems as a color-phase shift resulting from thermal expansion of the shadow mask.
However, the above-mentioned alloy sheet for a shadow mask manufactured from the INVAR alloy, i.e., a material sheet prior to the etching-piercing of passage holes for the electron beam (hereinafter simply referred to as the "holes") has the following problems:
(1) Poor etching pierceability:
Because of a high nickel content in the INVAR alloy, the INVAR alloy sheet has, during the etching-piercing, a poor adhesivity of a resist film onto the surface of the INVAR alloy sheet, and a poor corrosivity by an etching solution as compared with a low-carbon steel sheet.
This tends to cause irregularities in the diameter and the shape of the holes pierced by the etching, thus leading to a seriously decreased grade of the color cathode-ray tube.
(2) Easy occurrence of sticking of flat masks during annealing thereof:
An alloy sheet for a shadow mask as pierced by the etching, i.e., a flat mask, is press-formed into a curved surface to match with the shape of the cathode-ray tube. The flat mask is annealed prior to the press-forming in order to improve press-formability thereof. It is the usual practice, at cathode-ray tube manufacturers, to anneal several tens to several hundreds of flat masks made of the INVAR alloy which are placed one on the top of the other at a temperature of from 810.degree. to 1,100.degree. C., which is considerably higher than the annealing temperature of the flat masks made of the low-carbon steel, with a view to improving productivity.
Since the INVAR alloy has a high nickel content, it has a higher strength than a low-carbon steel. A flat mask made of the invar alloy must therefore be annealed at a higher temperature than in a flat mask made of a low-carbon steel. As a result, sticking tends to occur in the flat masks made of the INVAR alloy during the annealing thereof.
For the purpose of solving the problem (1) as described above, the following prior arts are known:
(a) Japanese Patent Provisional Publication No. 61-39,344 discloses limitation of the center-line mean roughness (Ra) of an alloy sheet for a shadow mask within a range of from 0.1 to 0.4 .mu.m (hereinafter referred to as the "prior art 1").
(b) Japanese Patent Provisional Publication No. 62-243,780 discloses limitation of the center-line mean roughness (Ra) of an alloy sheet for a shadow mask within a range of from 0.2 to 0.7 .mu.m, limitation of the average peak interval of the sectional curve representing the surface roughness within a standard length to up to 100 .mu.m, and limitation of the crystal grain size to at least 8.0 as expressed by the grain size number (hereinafter referred to as the "prior art 2").
(c) Japanese Patent Provisional Publication No. 62-243,781 discloses, in addition to the requirements disclosed in the above-mentioned prior art 2, limitation of Re, i.e., the ratio of .alpha..sub.1 /.alpha..sub.2 of the light-passage hole diameter (.alpha..sub.1) to the etching hole diameter (.alpha..sub.2) to at least 0.9 (hereinafter referred to as the "prior art 3").
(d) Japanese Patent Provisional Publication No. 62-243,782 discloses that the crystal texture of an alloy sheet for a shadow mask is accumulated through a strong cold rolling and a recrystallization annealing, the crystal grain size is limited to at least 8.0 as expressed by the grain size number, and the surface roughness described in the above-mentioned prior art 2 is imparted to the surface of the alloy sheet for a shadow mask by means of the cold rolling with the use of a pair of dull rolls under the reduction rate of from 3 to 15% (hereinafter referred to as the "prior art 4").
In order to solve the problem (2) as described above, on the other hand, the following prior art is known:
(e) Japanese Patent Provisional Publication No. 62-238,003 discloses limitation of the center-line mean roughness (Ra) of an alloy sheet for a shadow mask within a range of from 0.2 to 2.0 .mu.m, and limitation of the skewness (Rsk) which is a deviation index in the height direction of the roughness curve to at least 0 (hereinafter referred to as the "prior art 5").
However, the above-mentioned prior arts 1 to 4 have the problem in that while it is possible to improve etching pierceability of the alloy sheet to some extent, it is impossible to prevent the occurrence of sticking of the flat masks during the annealing thereof.
The above-mentioned prior art 5 has, on the other hand, a problem in that, while it is possible to prevent sticking of the flat masks made of the low-carbon steel during the annealing thereof to some extent, it is impossible to prevent sticking of the flat masks during the annealing thereof, made of the INVAR alloy which requires a higher annealing temperature than the low-carbon steel.